The present invention is directed generally to techniques for stabilizing mercury-containing materials and specifically to techniques for stabilizing elemental and speciated mercury in liquid and solid wastes.
Because of the low melting point of elemental mercury and the toxicity of elemental mercury and environmentally unstable forms of speciated mercury to animals and humans, many environmental regulatory agencies, such as the U.S. Environmental Protection Agency, restrict the disposal of elemental mercury and unstable forms of speciated mercury. Unstable forms of speciated mercury include mercury oxide and water soluble mercury compounds such as mercuric chloride, methylated mercury, and organomercury compounds. While the disposal of such forms of mercury remains a problem, hazardous waste materials containing mercury are being generated daily by many sources, such as laboratories that use unstable forms of mercury in testing procedures, manufacturers that build products containing unstable forms of mercury, and scrap products containing unstable forms of mercury such as thermometers, vacuum tubes, x-ray tubes, thermostats, and the like. The U.S. Department of Energy alone has over 5,000 kilograms of radioactively contaminated, liquid elemental mercury awaiting disposal.
A common method for stabilizing mercury-containing compounds is amalgamation with an amalgamating agent, such as lead, copper, silver, zinc, magnesium, aluminum, and sulfur. Methods using amalgamation commonly (a) are unable to stabilize adequately unstable forms of mercury in the hazardous waste material, particularly liquid elemental mercury waste materials or waste materials containing both elemental and unstable forms of speciated mercury, to pass applicable environmental regulations, (b) use expensive additives, and/or (c) operate on only a small scale and are difficult to scale up economically to handle large quantities of mercury-containing wastes.
There is therefore a need for a process for stabilizing mercury that effectively stabilizes both elemental and speciated mercury such that the treated waste material complies with pertinent environmental regulations, uses relatively inexpensive additives, and/or operates economically both on small and large scales.
The method and apparatus of the present invention provide a simple, scalable, inexpensive methodology for converting hazardous forms of mercury in waste materials into environmentally stable and nonhazardous waste materials. In one embodiment, the method includes the steps of:
combining the mercury-containing feed material with a polysulfide and a reactive sulfur-containing compound other than the polysulfide to form a composite feed material; and
mixing the composite feed material to react the reactive sulfur-containing compound with the elemental and/or environmentally unstable forms of speciated mercury to form mercuric sulfide. The process effectively stabilizes both elemental and environmentally unstable forms of speciated mercury such that the treated waste material complies with pertinent environmental regulations (i.e., can pass the Toxic Characterization Leach Procedure or TCLP, or contains preferably no more than about 1,000 ppm and more preferably no more than about 200 ppm elemental mercury and/or environmentally unstable forms of speciated mercury).
The feed material can include high or low concentrations of either environmentally unstable forms of speciated mercury such as mercury oxide and water soluble mercury compounds (e.g., mercuric chloride, methylated mercury and organomercury compounds), and/or elemental mercury. Typically, the feed material has a total content of elemental and unstable forms of speciated mercury broadly ranging from about 0.01 to about 100% by weight. The process can be highly effective in applications where the feed material contains small or large amounts of elemental mercury and little or no speciated mercury or small or large amounts of speciated mercury and little or no elemental mercury. For liquid elemental mercury in particular, the feed material typically contains at least about 50 wt % elemental mercury and commonly is radioactive.
Because many mercury contaminated wastes contain water, the reactive sulfur-containing compound is any sulfur-containing compound (other than a polysulfide) that reacts with mercury in aqueous solutions. More preferably, the compound is elemental sulfur, an inorganic sulfide, and/or mixtures thereof, and even more preferably elemental sulfur, an alkali metal hydrogen sulfide, a mercaptan, an alkali metal sulfide, or mixtures thereof, with elemental sulfur being even more preferred. Elemental sulfur is reactive with mercury and is readily available and inexpensive. The preferred molar ratio between the reactive sulfur-containing compound on the one hand and the elemental mercury and/or unstable forms of speciated mercury on the other ranges from about 1:1 to about 30,000:1 and more preferably from about 2:1 to about 100:1. The reactive sulfur-containing compound typically ranges from about 10 to about 50 wt % and more typically from about 20 to about 30 wt % of the composite feed material.
The reactive sulfur-containing compound is preferably added to the feed material in the form of a powder. The average particle size of the powder preferably ranges from about 10 to about 500 micrometers and more preferably from about 50 to about 100 micrometers.
The polysulfide acts as an activator of the reaction between the reactive sulfur-containing compound and the unstable forms of mercury and is preferably selected from the group consisting of calcium polysulfide, sodium polysulfide, and other alkaline earth polysulfides and mixtures thereof. Such forms of polysulfide are readily soluble/suspendable in water and are readily available and inexpensive. The composite feed material preferably includes from about 0.5 to about 20 wt % and more preferably from about 2 wt % to about 10 wt % of the polysulfide. The preferred molar ratio between the reactive sulfur-containing compound on the one hand and the polysulfide on the other ranges from about 3:1 to about 650:1 and more preferably from about 10:1 to about 150:1 and between the polysulfide on the one hand and the elemental mercury on the other and unstable forms of speciated mercury preferably range from about 2000:1 to about 0.01:1 and more preferably from about 100:1 to about 0.1:1.
The polysulfide is typically dissolved or suspended in a liquid carrier or solvent. The preferred liquid carriers/solvents is water. A particularly preferred solution comprises from about 5 to about 29 wt % polysulfide. The polysulfide additive includes preferably from about 70 to about 95% and more preferably from about 75 to about 95% by weight of the liquid carrier/solvent.
The composite feed material is typically in the form of a flowable liquid (e.g., a nonviscous liquid or a viscous liquid such as a paste or slurry) and has a pH preferably at least about pH 7 and more preferably from about pH 9 to about pH 12 to inhibit the release of H2S gas during the mixing step.
The duration of the mixing step is important to realize the substantially complete conversion of the elemental mercury and speciated mercury to mercuric sulfide. Preferably, the mixing step has a duration of at least about 30 minutes and more preferably from about 60 to about 120 minutes.
The mixing step is preferably performed by intrusive mixing techniques. The mixing step can be performed by any suitable mixing device capable of mixing a viscous material and expelling (or venting) any vaporized liquid, such as by blending, beating, grinding, and the like, with stirred vessels including a plurality of rotating mixing blades being more preferred. The mixer is preferably at least one of the following: a pug mill, a screw-type mixer, a planetary mixer, and a ribbon blender. In such mixers, the blades preferably rotate at least about 5 rpm and most preferably at from about 10 to about 200 rpm. The mixer can be designed to be continuous by providing the mixer with a sufficient length to provide the required residence time.
A liquid and/or bulking agent can be combined, alone or together, with the feed material, to control the temperature of the intermediate feed material during the mixing step. As will be appreciated, the reaction of elemental sulfur and other mercury-reactive materials with the mercury is spontaneous and highly exothermic at room temperature, and the rate of the reaction is temperature dependent. The release of thermal energy from the reaction can increase the operating temperature and therefore the reaction rate which in turn releases additional thermal energy and so on which can ultimately lead to an uncontrolled reaction.
The liquid is typically selected such that the boiling point of the liquid is at or near the desired maximum operating temperature of the system to assist in controlling the operating temperature. Preferably, the maximum desired operating temperature is about 120xc2x0 C. or less and more desirably about 100xc2x0 C. or less. The preferred liquid for temperature control is water. Preferably, sufficient water is contacted with the feed material to provide a total water content of the composite feed material ranging from about 5 to about 30 wt. %.
The bulking agent provides not only temperature control but also a sufficient volume for efficient mixing to take place. The bulking agent can be any microporous granulated material, such as soils, sand, cement, vermiculite, perlite, silica gel, clays, plant fibers (ground corn cobs, sawdust, etc.), zeolites, activated carbon, activated alumina, polyacrylamide, and mixtures thereof, with sand, activated carbon, and activated alumina being most preferred. Preferably, the composite feed material contains from about 0-50 wt % and more preferably from about 20 wt % to about 40 wt % of the granulated material. The volumetric ratio of the bulking agent added to the feed material preferably ranges from about 0:1 to about 10:1 and preferably from about 1:1 to about 4:1. The average particle size of the bulking agent is preferably no more than about xe2x85x9th inch.
The order of addition of the various components is important. Preferably, the order of addition is as follows: (a) the reactive sulfur-containing compound is first added to mixer and allowed to coat the inner surfaces of the mixer; (b) the feed material is next added to the mixer and is mixed with the reactive sulfur-containing compound to form a first composite feed material; (c) the bulking agent is then added to and mixed with the first composite material to form a second composite feed material; and (d) finally the polysulfide is added to and mixed with the second composite feed material to form the composite feed material. Water may be added at any time during these steps to mitigate the formation of dust and to control temperature.
In a second embodiment of the present invention, a method for stabilizing elemental and/or speciated mercury in a feed material is provided that includes:
combining the mercury-containing feed material with a bulking agent, a polysulfide, and an mercury-reactive material to form a composite feed material; and
intrusively mixing the composite feed material to react the mercury-reactive material with the elemental and/or speciated mercury to form a mercury-containing compound or amalgam.
As used herein, mercury-reactive material broadly refers to any element or compound that forms an environmentally stable compound with mercury. The mercury-reactive material can be any substance that forms an environmentally stable compound or amalgam with mercury such as an environmentally benign metal (i.e., lead, silver, gold, zinc, magnesium, copper, aluminum, and reactive sulfur-containing compound. By way of example, the more preferred mercury-reactive material, namely mercaptans, alkali metal sulfides, alkali metal hydrogen sulfides, elemental sulfur, and mixtures thereof (with elemental sulfur being even more preferred), form an environmentally stable compound, HgS, when reacted with elemental mercury. The mercury-reactive material can be added in the form of a solid (e.g., a powder) or a solution in which the mercury-reactive material is dissolved in a solvent. Preferably, for liquid elemental mercury as the feed material, the composite feed material includes at least about 10 wt % of the mercury-reactive material (based upon 100 wt % mercury in the feed material) and most preferably from about 20 to about 30 wt % of the mercury-reactive material (based upon 100 wt % mercury in the feed material). The mass ratio of the mercury-reactive material to the mercury in the intermediate feed material preferably ranges from about 0.3:1 to about 1000:1 and more preferably from about 0.5:1 to about 100:1.
The method and system have a number of benefits. The process can be performed using conventional mixing devices on a batch, semi-continuous or continuous basis. The process is therefore readily scalable using existing (conventional) equipment.